This invention relates generally to the treatment of sesquisulfate waste streams to recover acid and sulfate values therefrom. More particularly, the invention relates to an apparatus and method for treating a sodium sesquisulfate (Na3H(SO4)2)-containing waste stream from a chlorine dioxide production process to recover a sulfuric acid (H2SO4) solution and sodium sulfate solids (Na2SO4).
Bleaching or whitening of pulp is typically accomplished by chemically altering and/or removing colored matter in the pulp and imparting a higher brightness thereto. Chlorine-based chemicals such as chlorine, chlorine dioxide and hypochlorite have been used in pulp bleaching for many years and continue to be used for removing lignin and bleaching the pulp to high brightness. Growing environmental concerns have led to an increase in the use of chlorine dioxide and nonelemental chlorine agents such as oxygen, peroxide and/or ozone in bleaching processes.
Waste streams containing sodium sesquisulfate result from the production of chlorine dioxide for use in bleaching pulp. The increasing use of chlorine dioxide in pulp mills thus makes recovery of the acid and sulfate values of such waste streams of considerable economic importance since the components recoverable from the waste streams may be recycled for a variety of uses.
One process for recovering sodium sulfate and sulfuric acid from a stream containing sodium sesquisulfate is described in U.S. Pat. No. 5,116,595 and is commonly referred to as the xe2x80x9cR10xe2x80x9d process. The ""595 patent is specifically directed to treatment of a waste stream obtained from a process for generating chlorine dioxide for use in the pulp mill commonly known in the art as the xe2x80x9cR8xe2x80x9d process, and described in U.S. Pat. No. 4,081,520.
In accordance with the method described in the ""595 patent, a slurry containing sulfuric acid and sodium sesquisulfate is filtered, contacted with water, and thereafter filtered again to separate the resulting sodium sulfate precipitate from the sulfuric acid solution. One significant disadvantage of recovery processes of this type is that they require filtration techniques, typically vacuum filtration techniques, and the concomitant equipment and operational costs associated with these filtration steps. However, such filtration techniques have been considered vital to achieving sulfate solids of satisfactory yield.
Accordingly, it is an object of the present invention to provide a method and apparatus for treating a sesquisulfate-containing stream to recover valuable constituents thereof.
It is an additional object of the invention to provide an apparatus and method for treating a sesquisulfate stream to recover acid and sulfate values of the stream.
Another object of the invention is to provide an apparatus and method for treating a sodium sesquisulfate-containing waste stream to recover a sulfuric acid solution and a sodium sulfate solids.
A further object of the invention is to provide an apparatus and method of the character described which avoids or limits the need for mechanical filtration such as vacuum filtration.
Still another object of the invention is to provide an apparatus and method of the character described which is uncomplicated and economical as compared to conventional methods and which does not compromise the quality or quantity of the yield.
With regard to the foregoing and other objects, the invention provides an apparatus for treating a slurry containing sodium sesquisulfate crystals to recover sulfate and acid constituents therefrom. In general, the apparatus includes an upright treatment vessel having an upper end and a lower end and a separation wall defining on one side a mixing zone and on another side a clarifying zone. The separation wall includes a lower edge spaced above the lower end of the vessel to enable fluid flow communication between the mixing zone and the clarifying zone by flow of liquid under the lower edge between the zones.
A collector is located within the treatment vessel generally below the mixing zone and includes an upwardly opening reservoir for collecting solid particles descending by gravity from the mixing zone. A diffuser in the reservoir of the collector is connected in flow communication with a source of pressurized gas for releasing a flow of gas bubbles through solid particles collected in the reservoir to agitate particles contained therein, and to provide a flow of gas bubbles up through the mixing zone to promote a turbulent mixing flow regime within the mixing zone for dissolution of sodium sesquisulfate crystals and formation of sodium sulfate solids.
An inlet conduit is connected in flow communication with the mixing zone for delivering the slurry into the mixing zone. A water inlet is provided for introducing water into the mixing zone for mixing with the slurry.
An outlet conduit adjacent the upper end of the treatment vessel is provided for directing liquid from the clarifying zone to a conventional liquid processing unit. A solids outlet is provided adjacent the lower end of the treatment vessel for directing sodium sulfate solids out of the vessel.
In a preferred embodiment, the separation wall is provided by an upright elongate cylindrical conduit generally centrally located within the treatment vessel. The mixing zone is defined within the conduit as an elongate upright cylindrical columnar space and the clarifying zone is defined outside of the conduit as an elongate upright annular space between the conduit and the inner wall of the vessel. The lower edge of the wall is defined as the lower continuous edge of a generally conical, downwardly opening flow expansion member on the conduit.
The invention also provides a method for treating a slurry containing sodium sesquisulfate crystals to recover a sodium sulfate solids therefrom. In accordance with the method, the slurry is conducted through an elongate generally upright mixing zone in a treatment vessel and is mixed therein to promote dissolution of the crystals and formation of sodium sulfate solids. The slurry is then conducted to a clarifying zone.
Sesquisulfate crystals descending from the mixing zone are collected in the reservoir of a collector located beneath the mixing zone. Gas is bubbled through the crystals in the reservoir to promote dissolution thereof and to provide a flow of gas bubbles ascending through the mixing zone to promote a turbulent flow regime therein and consequent mixing of the slurry. Sodium sulfate solids descending from the slurry in the clarifying zone are collected and conducted out of the vessel along with entrained liquid consisting principally of sulfuric acid and water. The balance of the liquid from the clarifying zone is conducted out of the vessel along with a small amount of entrained solids consisting principally of sodium sulfate.
Treatment of a sesquisulfate-containing stream in accordance with the invention yields an effluent containing sulfuric acid, water and a small amount of dissolved sodium sulfate and a sodium sulfate solids product with entrained sulfuric acid containing little or no sesquisulfate crystals, limiting the need for vacuum filtration and other expensive, complicated separation techniques.